The present invention relates to a sensor for non-contact torque measurement on a shaft having a measurement layer support, which has a first end region for torque-coupled fixing on a shaft, and a second end region which is an annular disc and extends essentially at right angles to an axis of the shaft when the measurement layer support is fitted, a downstream element and a cylindrical surface being torque-coupled fixed between the first and second end regions, on which cylindrical surface a soft-magnetic and magnetostrictive measurement layer is arranged which is coaxial with respect to the shaft when the measurement layer support is fitted, and a measurement coil support, fitted with a measurement coil that produces an electrical output signal as a function of a mechanical stress state of the measurement layer, the measurement coil being coaxially opposite the surface of the measurement layer. The invention also relates to a soft-magnetic, magnetostrictive measurement layer which can be used for this sensor.
A sensor of the above-described type is disclosed, for example, in European Patent Document 0 285 259 B1. The measurement layer support in that sensor consists of a specially constructed tubular measurement shaft piece whose end regions can be fixed on associated shaft ends of the shaft, which is cut through for this purpose. The measurement coil support is fitted with an excitation coil to which alternating current is applied and a detector coil, which detects the torque-dependent permeability changes in the measurement layer and is supported by a ball bearing on a region of the measurement layer support which extends axially between the region occupied by the measurement layer and the associated end region, which forms a radially projecting annular flange.
German Patent Document 36 35 207 C2 discloses a sensor for non contact torque measurement on a shaft, in the case of which the measurement layer is part of a layer sequence arranged firmly on the shaft and is magnetically excited by a permanent magnet. Provided as the measurement arrangement is a U-shaped support which is composed of a magnetic, non-magnetostrictive metal and whose web is fitted with a measurement coil so that the permeability changes induced in the measurement layer by the changes in the torque are detected as changes in the inductance of the measurement coil which, for this purpose, is looped into a circuit which detects its inductance. The measurement layer which is used consists mainly of nickel, a specific component of phosphorus being added in order to control the magnetic properties of the layer.
German Patent Document 34 07 917 A1 specifies as the material for the measurement layer of a torque sensor a material whose chemical composition is M.sub.y X.sub.100-y, M being at least one of the metals iron, cobalt and nickel, X being at least one of the elements boron, carbon, silicon and phosphorus and, furthermore, y being between approximately 70% and 90%. In this case, in addition to the metals, a component of up to 4% by atomic weight of at least one of the metals chromium, molybdenum, tungsten, vanadium, niobium, tantalum, titanium, zircon, hafnium and manganese can be provided in addition to the metals and, moreover, the elements aluminum, gallium, indium, germanium, tin, lead, arsenic, antimony, bismuth or beryllium can be provided in addition to or instead of the individual components X. Alloys having a majority component of cobalt are recommended there as being preferred.
An object of the present invention is to provide a sensor for non-contact torque measurement on a shaft having a structurally simple, robust and physically small structure which, in particular, also allows the measurement coil to be replaced easily, and a measurement layer which is suitable for such a sensor.
This and other objects are achieved by the present invention which provides a sensor for non-contact torque measurement on a shaft comprising a measurement layer support, which has a first end region for torque-coupled fixing on a shaft, and a second end region which is an annular disc and extends essentially at right angles to an axis of the shaft when the measurement layer support is fitted. A downstream element and a cylindrical surface are torque-coupled fixed between the first and second end regions. On the cylindrical surface a soft-magnetic and magnetostrictive measurement layer is arranged which is coaxial with respect to the shaft when the measurement layer support is fitted. A measurement coil support is provided and is fitted with a measurement coil that produces an electrical output signal as a function of a mechanical stress state of the measurement layer, the measurement coil being coaxially opposite the surface of the measurement layer. A holder on which the measurement coil support is fixed without being supported on the measurement layer support is also provided. The axial structural length of the measurement layer support substantially corresponds to the axial extent of the measurement layer, and the second end region of the measurement layer support is located in the axial region occupied by the measurement layer.
The sensor of the present invention can be manufactured with a very short axial structural length and its measurement coil support can be replaced easily since it is not mechanically coupled to the measurement surface support. There is no need for any ball bearing between the measurement coil support and the measurement surface support. Nevertheless, the sensor is mechanically robust and is especially advantageous in a rugged physical and chemical environment. The sensor principle enables the use of a simple electronics unit which detects the changes in the coil inductance. In addition to nickel as a component, the measurement layer of the present invention contains in certain embodiment a phosphorus addition and, in certain preferred embodiments, small components of a further element in the main group IV or V, especially antimony, as well as a transition metal, especially cobalt. The components of the elements in the main group IV or V in this case produces a considerable increase in the electrical resistivity as a result of the high component of atom bonds, so that particularly efficient damping of eddy currents is provided for the measurement layer. The transition metal component produces an increase in the crystallization temperature of the measurement layer and in the Curie temperature of nickel to a value of more than 700 Kelvin in each case. An electrolyte which is used to produce the measurement layer contains a compound, such as saccharin for example, which influences inherent stress. It has been found that such an addition of saccharin influences the inherent stress of the measurement layer in a favorable manner and thus contributes to increasing the magneto-elasticity.
In certain advantageous embodiments of the invention, the measurement coil is looped into a circuit which detects its inductance, the measurement coil at the same time representing the element which produces magnetic fields and measures the magnetic field, so that no further magnetic fields producing elements need be provided.
Particularly advantageous embodiments provide that the axial structural length of the overall sensor arrangement is essentially only as large as the width of the measurement layer. A positive influence on the measurement accuracy is provided by certain embodiments in which the reference layer allows all those changes in the reference layer to be detected which are not governed by torque, because the sleeve as a separate component is not, as a function of the design, subject to the power flow of the torque transmission, so that these influences which are not governed by the torque can be extracted from the measurement coil signal. Since, in addition, the reference coil is arranged coaxially with respect to the measurement coil, the axial structural length in not increased by this measure.
When the reference coil is provided, this reference coil is preferably insulated electrically and magnetically from the measurement coil by the sequence of three layers provided in certain embodiments, in order to prevent the two coils influencing one another in an interfering manner.
In certain embodiments of the composition of the measurements layer, an electrolyte which is used for producing the measurement layer contains compounds such as saccharin, for example, which influence the inherent stress. It has been found that such an addition of saccharin influences the inherent stress of the measurement layer in a favorable manner and thus contributes to increasing the magneto-elasticity.